The present invention is concerned with the transfer of heat from a first body of fluid to a second body of fluid. The obvious way to achieve this is merely by replacement of the second fluid by the first heated fluid. However, replacement is not always desirable or even possible.
For example, in a solar water heating system a tank or reservoir of cool or cold water is conveniently located as in a building and is connected to one or an array of solar collectors disposed adjacent or on the roof of the building. In the known and conventional solar water heating system a pump is utilized to pump cold water from the tank to and through the collectors. While in the collectors, the water will become heated by solar energy and will then flow back into the tank, displacing some of the cold water.
Such a system poses several problems. For example, in cold weather, especially at night, the collectors are not effective to heat the water and, therefore, the water must be drained to prevent freezing. To overcome this problem antifreeze solutions are often added to the water. However, this pollutes or contaminates the water and greatly limits the uses to which the water may be put.
Certain state and local codes require that at least two barriers be maintained between the nonfreezing agent and potable water. Therefore, it has been contemplated that the water in the solar collector loop may be used to heat a separate body of water, such as drinkable tap water, by disposing the tap water in a tank or reservoir and surrounding or enclosing the tank within the heated water which may be contained as in a double-walled metal wrap-around heat exchanger. By virtue of the physical contact between the wrap-around heat exchanger and the tank, heat from the water in the heat exchanger will pass into the water in the tank.
Such heat exchangers, however, are expensive and somewhat inefficient because the adjacent surfaces of the heat exchanger and tank do not readily conform, thus producing losses in heat transfer performance. This problem has been at least partially overcome by the device disclosed in U.S. patent application Ser. No. 925,025, filed July 17, 1978 by Hapgood and Knight, Jr. and assigned to the same assignee as the present invention . The device in said application comprises a wrap-around heat exchanger in the form of a hollow plastic body shaped similar to a mattress and having convolutions through which the heated fluid passes. However while such heat exchangers are practical and functional, they also are relatively difficult to fabricate and, consequently, comparatively expensive.